In addition to heavy and light chains, human and animal polymeric immunoglobulins contain a polypeptide described as J chain. It is a small molecular weight (15,600 plus or minus 200) glycoprotein (7.6% carbohydrate) attached to the Fc fragment of the molecule by disulfide bond(s). J chain can be prepared from IgA of patients with multiple myeloma. J chain in a free form has not been found in urinary proteins, saliva, colostrum or serum. In patients with multiple myeloma, where IgA is present in a polymeric form with J chain attached, the clinical course of the disease is complicated by the presence of hyperviscosity syndrome. In human polymeric IgA and IgM, J chain is linked to the penultimate cysteine residue of one or two H chains. Therefore, J chain cannot join all IgMs in polymeric IgM. Secretory component of human S- IgA is linked to the alpha chain directly and not through J chain. These data indicate that J chain is involved indirectly in the process of polymerization and SC binding and probably mediates disulfide-bond- interchange reactions. In patients with myelomatosis, J chain is produced within the same cell as immunoglobulins. Comparative studies that include J chains prepared from human IgA and catfish macroglobulin will be performed. Isolation procedures, molecular weight, electrophoretic behavior, amino acid and carbohydrate compositions, and antigenic determinent will be studied. Quantitation of J chain and individual variations will be investigated in human polymeric immunoglobulins.